python3 study notes custom class

__str__

class Student(object):
    def __init__(self, name):
        self.name = name
    def __str__(self):
        return 'Student object (name: %s)' % self.name
　　__repr__ = __str__ #Function 

: Internal information can be printed directly

 >>> print (Student( ' Michael ' ))
Student object (name: Michael)

 

__getattr__

When calling an attribute that doesn't exist, for example score, the Python interpreter will try to call __getattr__(self, 'score')to try to get the attribute, so we have a chance to return scorethe value:

class Student(object):

    def __init__(self):
        self.name = 'Michael'

    def __getattr__(self, attr):
        if attr=='score':
            return 99

>>> s = Student()
>>> s.name
'Michael'
>>> s.score
99

 

#It is also perfectly possible to return a function: 
class Student(object):

    def  __getattr__ (self, attr):
         if attr== ' age ' :
             return  lambda : 25 The 
call method should be changed to: 
>>> s.age() 
25

Any call s.abcwill return None, this is because the __getattr__default return we defined is None. To make the class respond to only a few specific properties, we have to throw AttributeErroran error according to the convention:

class Student(object):

    def __getattr__(self, attr):
        if attr=='age':
            return lambda: 25
        raise AttributeError('\'Student\' object has no attribute \'%s\'' % attr)

This can actually process all the properties and method calls of a class dynamically without any special means.

Example of action:

With fully dynamic __getattr__, we can write a chained call:

class Chain(object):

    def __init__(self, path=''):
        self._path = path

    def __getattr__(self, path):
        return Chain('%s/%s' % (self._path, path))

    def __str__(self):
        return self._path

    __repr__ = __str__ 




>>> Chain().status.user.timeline.list ' 
/status/user/timeline/list' 
In this way, no matter how the API changes, the SDK can implement fully dynamic calls according to the URL, and it does not follow the API Increase and change!

__call__

Any class, just need to define a __call__()method, you can directly call the instance.

class Student(object):
    def __init__(self, name):
        self.name = name

    def  __call__ (self):
         print ( ' My name is %s. ' % self.name) 


is called as follows: 
>>> s = Student( 'Michael') 
>>> s() # Do not pass 
My name to the self parameter is Michael.